1. Field of the Invention
The present invention relates to analog-to-digital converters (ADCs), and more particularly, to phase synchronization between multiple ADCs.
2. Statement of the Problem
Vibrating conduit sensors, such as Coriolis mass flowmeters and vibrating densitometers, typically operate by detecting motion of a vibrating conduit that contains a flowing material. Properties associated with the material in the conduit, such as mass flow, density and the like, can be determined by processing measurement signals received from motion transducers associated with the conduit. The vibration modes of the vibrating material-filled system generally are affected by the combined mass, stiffness and damping characteristics of the containing conduit and the material contained therein.
A typical Coriolis mass flowmeter includes one or more conduits that are connected inline in a pipeline or other transport system and convey material, e.g., fluids, slurries, emulsions, and the like, in the system. Each conduit may be viewed as having a set of natural vibration modes, including for example, simple bending, torsional, radial, and coupled modes. In a typical Coriolis mass flow measurement application, a conduit is excited in one or more vibration modes as a material flows through the conduit, and motion of the conduit is measured at points spaced along the conduit. Excitation is typically provided by an actuator, e.g., an electromechanical device, such as a voice coil-type driver, that perturbs the conduit in a periodic fashion. Mass flow rate may be determined by measuring time delay or phase differences between motions at the transducer locations.
Two such transducers (or pickoff sensors) are typically employed in order to measure a vibrational response of the flow conduit or conduits, and are typically located at positions upstream and downstream of the actuator. The two pickoff sensors are connected to electronic instrumentation. The instrumentation receives signals from the two pickoff sensors and processes the signals in order to derive a mass flow rate measurement, among other things.
The pickoff signals typically comprise time-varying analog signals that are generated by the vibrations as picked up by pickoff sensor coils. The analog pickoff signals are subsequently converted into digital signals for processing.
FIG. 1 shows a prior art analog-to-digital converter (ADC) arrangement for a Coriolis flow meter. The left pickoff signal generated by the left pickoff (LPO) is fed into a first ADC and the right pickoff signal generated by the right pickoff (RPO) sensor is fed into a second ADC. Each ADC digitizes a respective analog signal and outputs a corresponding digitized pickoff signal to a processor or other circuitry for further processing. For example, a processing can comprise determining a phase difference between the pickoff sensor signals due to the Coriolis effect. A phase difference can be used to determine a mass flow rate through the meter.
It is obvious that any phase differences introduced by the two ADCs will be seen by the processor and will negatively affect a mass flow rate measurement. The phase drift performance of an ADC is inherent in its design. The phase drift of an ADC can be affected by input signal amplitude changes, power supply voltage changes, EMC effects, temperature changes, input frequency changes, noise content, harmonic content, or other reasons.
Commercially and readily available is a component called a codec, comprising a combination coder-decoder. A codec contains two ADCs on the same silicon die, along with digital-to-analog converters (DACs), with these components typically being designed to work in the audio range (i.e., at frequencies up to about 20,000 Hz). It is difficult to find a suitable codec for dual channel analog-to-digital conversion where phase difference is important, as different manufacturers have different construction processes which can result in very different phase performance. Additionally, codecs are designed to work in the audio world, where small phase differences are acceptable. Therefore, commercially available codecs do not have any phase specifications available for use in Coriolis flow meter design. Further, since the phase drift is not specified, manufacturer can change the phase drift performance at any point. This puts a user in the position of having to keep checking the phase drift performance.